You can extract the Skull from a body, which can later be used for many things. However, when you first meet the Astrologer, he requires one from you.
The only way that the Astrologer will actually help you and answer any questions regarding the portal is if you give him a skull. He doesn’t want Gerry the talking skull, instead he wants a normal one from a corpse.
This guide will require you to unlock a node from the technology tree. You can see our guide below for getting skulls. Whilst we have a whole collection of Graveyard Keeper guides available.
Collecting Skulls
First off, you need to unlock the Hardspares node in the technology tree. It costs 10 red points 5 green points. It allows you to extract the skull, bones and skin during autopsies.

You will either need to exhume a body, or wait for one to come in on the cart. Take it down to the morgue, put it on the operating table and conduct an autopsy. You’ll see a screen similar to below, with a few new options.

As the astrologer wants the skull, choose to extract it from the body. Once done, you can expose of the body in any way you wish.